Determination whether a blood vessel is occluded (i.e. blocked) or patent (i.e. open), and, in the case of an occlusion being identified, localization of this occlusion, are important in the diagnosis and treatment of vascular diseases. This is also critical in acute stroke management. Imaging is a means to obtain this information. Volumetric angiographic data (that is, a set of data in which elements of the data correspond to respective points within a three-dimensional region) can be acquired with various imaging techniques, such as magnetic resonance angiography (MRA), computed tomography angiography (CTA) or X-ray rotation angiography. Present techniques for detecting occlusions involve a requirement to segment volumetric angiographic images (i.e. partition the images into regions which are the estimates of the positions of cerebral structures within the image). For this purpose, the Marching Cubes algorithm is typically used. The whole procedure of iso-surface segmentation and surface display is time consuming, particularly, in acute stroke management. Alternatively, the scan can be examined by using Maximum Intensity Projection (MIP) display. MIP, as a 2D projection of a 3D volume does not contain full information.